Since 1992 Puerto Rico has been participating in the National Duchenne/Becker Muscular Dystrophy Screening program sponsored by the Muscular Dystrophy Association (MDA) and the University of Pittsburgh Department Of Genetics. In this project all newborns with persistently elevated Creatine Kinase (CK) are tested for a deletion in the Dystrophin molecular gene. This deletion, if present, is diagnostic for Duchenne/Becker Muscular Dystrophy. To this date, the Puerto Rico, State- Sponsored, Neonatal Screening Program has processed more than 120,000 blood samples screening over 97% of all the newborns in the Island. This has been the most complete sample of the national screening program so far. However, a number of patients with neonatally elevated CK remain without a diagnosis. In our proposed study, these cases, currently registered and identified by the Duchenne/Becker screening project, will undergo the necessary evaluations to reach a final diagnosis. With the results of these diagnostic studies we will be able to determine: a) the different etiologies of neonatal CK elevation in a Hispanic population, b) the prevalence at birth of specific myopathies affecting our population and c) the prevalence at birth of hereditary versus new mutation cases of Duchenne/Becker muscular dystrophy in a Hispanic population. This study will establish the basis for the diagnostic work-up of elevated CK in Hispanic newborns. It will also create a population based-registry of patients diagnosed early in life which can be available for future studies, including drug trials and gene therapy. The study will also provide information on the molecular genetics of Duchenne/Becker and other myopathies in a Hispanic population.